a. Field of the Invention
The present invention relates to a centrifugal concentrating machine and in particular to a screw decanter type centrifugal concentrating machine.
b. Description of Prior Art
Such decanter type centrifugal concentrating machine has a screw conveyor which is coaxially disposed within a rotary bowl which is rotated at a high speed. The rotary bowl and the screw conveyor are rotated in the same direction at different speeds. Liquid to be treated which is charged into the rotary bowl is sedimented on the inner wall of the rotary bowl by a centrifugal force and is separated into clarified liquid and concentrated liquid. The concentrated liquid is scraped toward one side of the machine by the screw conveyor and is discharged via a concentrated liquid discharge port while the clarified or separated liquid is discharged outside via a separated water discharge ports at the opposite side of the machine.
Various mechanisms for discharging the concentrated liquid and the separated liquid (both referred to as liquid to be discharged) have heretofore been developed. One of them is disclosed in, for example, Japanese Examined Patent Publication No. Sho 63-31261.
In the prior art, it is necessary to provide the liquid to be discharged with a high circumferential speed since the liquid to be discharged should be discharged via a discharge port formed on the outer periphery of the rotary bowl. The disclosed invention aims at eliminating the necessity to rotate the rotary bowl and the screw conveyer at a high speed in the prior art. At this end, a plurality of skimmer tubes are secured to the end face of the hub of a screw conveyer. The skimmer tubes extend in a radial direction so that their open ends are close to the inner wall of the rotary bowl to lead the concentrated liquid which has been scraped by the screw blades toward the side of the hollow supporting shaft of the rotary bowl through the skimmer tubes.
A plurality of radial guide passages which are formed on the hub of the screw conveyer, open on the outer surface of the hub to communicate with the hollow supporting shaft of the screw conveyer. The separated liquid is discharged outside via the hollow supporting shaft. The concentrated liquid is discharged outside through a space between the supporting shafts of the screw conveyer and the rotary bowl. A port of the discharge passage in the supporting shaft of the screw conveyer is provided with a flow rate adjusting means for adjusting the flow rate of the discharged separated liquid.
Other apparatus for discharging concentrated liquid and the like from an axial portion in order to reduce consumed power are disclosed in Japanese Unexamined Patent Publication No. Tokkai-Sho 62-45363, Japanese Unexamined Utility Model Publication Nos. Jikkai-Sho 62-136237, Jikkai-Hei 2-86652 and 2-86653.
Although the apparatus which is disclosed in Japanese Examined Patent Publication No. Sho 63-31261 in which the liquid to be discharged is discharged via a supporting shaft has an advantage in that the consumed power is less, wear of the inner wall surface of skimmer tubes through which concentrated liquid containing much fine solid content flows is remarkable and clogging is liable to occur since the skimmer tubes and guide passages which extend in a radial direction have a constant cross section in a radial direction and are narrow. Since the inlet ports of the skimmer tubes are narrow, dead spaces are liable to be formed. The concentration of the discharged concentrated liquid changes with the lapse of time.
Reduction in the diameters of the skimmer tubes to provide a compact machine will worsen the above mentioned problem. Accordingly, small size centrifugal machines can not be provided.